Initial version

Create two http client libraries. Both are packaged with shared HTTPClientCore, which provides common types and helpers such as HTTPRequestBuilder.
Third library is a test support framework that mainly helps on unit testing and mocking the clients.

HTTPClient

A tiny wrapper around URLSession that makes it easier to do simple REST networking.

CombineHTTPClient

Provides a Promise-like API built on top of URLSession, Combine and its Foundation conveniences.

HTTPClientTestSupport

Helpers and mocks that can help on testing code that uses one of the clients.